1. Field of the Invention
The present invention relates to a method of and an apparatus for forming a thin film for a semiconductor device, and more specifically, it relates to a method of and an apparatus for forming a thin film by chemical vapor deposition (hereinafter referred to as CVD) on a substrate, such as a semiconductor substrate or an insulating film provided on a semiconductor substrate, of a semiconductor device through an intermediate layer.
2. Description of the Background Art
In general, a wiring pattern for a semiconductor device is mainly formed of aluminum. In such a pattern of aluminum, however, projections called "hillocks" are grown in heat treatment after patterning, to cause electrical shorting across wires and break interlayer isolation films. Further, the temperature for heat treatment performed after formation of the wiring pattern must be limited to about 450.degree. C. since aluminum has a low melting point of about 600.degree. C. Thus, a problem is caused in manufacturing.
In recent years, therefore, metals having high melting points or silicides of such metals have been watched as wiring materials in substitution for aluminum. Among such materials, tungsten (W) has been positively subjected to development as a material having a high melting point of 3370.degree. C. and low specific resistance (bulk) of 5.6 t ; cm. To this end, an attempt has been made to form a thin film of tungsten by CVD, which is excellent in step coverage.
A thin film of tungsten may be formed through CVD by a process of reducing tungsten hexafluoride (WF.sub.6) with hydrogen (H.sub.2) or a process of reducing WF.sub.6 with silane (SiH.sub.4). Reaction formulas of such reduction processes are as follows: EQU WF.sub.6 (g)+3H.sub.2 (g).fwdarw.W (s)+6HF (g) (1) EQU 2WF.sub.6 (g)+3SiH.sub.4 (g).fwdarw.2W (s)+3SiH.sub.4 (g)+6F.sub.2 (g)(2)
where (g) and (s) represent gaseous and solid phases respectively. The process of reducing WF.sub.6 with H.sub.2 is superior in step coverage to the process of reducing WF.sub.6 with SiH.sub.4. However, H.sub.2 is inferior to SiH.sub.4 in ability for reducing WF.sub.6, and hence the H.sub.2 reduction process is inferior in deposition rate and nucleation power to the SiH.sub.4 reduction process. Therefore, it is necessary to increase the deposition temperature in the H.sub.2 reduction process. If the deposition temperature is increased, however, crystal grain size is excessively enlarged. Further, difficulty of nucleation causes deterioration of surface morphology and inferior homogeneity of film thickness within the surface plane. Therefore, it is usually difficult to manufacture W-CVD according to the process of reducing WF.sub.6 with H.sub.2. Further, W-CVD has difficulty in adhesive properties.
A thin film of tungsten formed by CVD is inferior in adhesion to an insulating film. In general, therefore, an intermediate layer is provided on a substrate or an insulating film, so that the tungsten thin film is formed on the intermediate layer. This intermediate layer is adapted to facilitate adherence of the tungsten thin film. Such an intermediate layer is prepared from an Si material such as polysilicon (poly-Si), a silicide material of a metal having a high melting point such as tungsten silicide (WSi.sub.x), a nitride of a metal having a high melting point such as titanium nitride (TiN), or the like.
When polysilicon is employed, however, it is necessary to dope an impurity corresponding to a junction layer of each contact, so that the steps are complicated. Further, the deposited tungsten thin film is inferior in surface morphology.
Also when tungsten silicide is employed, the tungsten thin film is inferior in surface morphology.
As compared with the above cases of polysilicon and tungsten silicide, a tungsten thin film deposited on an intermediate layer of titanium nitride has the advantage of being superior in surface morphology and excellent in adhesion. Further, titanium nitride has excellent properties as an underlayer for a CVD-W film since the same serves as a barrier metal which can be in contact with both of P-type and N-type layers.
However, it has been very difficult to form a tungsten film on such an intermediate layer of TiN by the conventional CVD process through H.sub.2 reduction, due to problems of the deposition rate, homogeneity of film thickness, surface morphology and the like, in particular.
Japanese Patent Laying-Open Gazette No. 63-250463 discloses a method of introducing a mixed gas of WF.sub.6 and an inert gas, or a mixed gas of WF.sub.6 gas and H.sub.2 gas, for depositing a tungsten thin film to a prescribed thickness and thereafter introducing a gas containing WF.sub.6 gas and a silane-system reducing gas for reducing the WF.sub.6 gas with the silane-system reducing gas, thereby growing a metal thin film. The tungsten thin film initially formed by introduction of the mixed gas of WF.sub.6 gas and the inert gas or the mixed gas of WF.sub.6 gas and H.sub.2 gas has an excellent contact property with both of N-type and P-type diffusion layers. Further, the tungsten film grown by the WF.sub.6 gas and the silane-system reducing gas in the second step is deposited at a high rate and hence the same can be increased in thickness. According to the method disclosed in the above gazette, therefore, it is possible to implement a tungsten wiring film which has an excellent contact property with both of N-type and P-type diffusion layers with no leakage current.
However, even if a tungsten film is formed on an intermediate layer of TiN by the method disclosed in the aforementioned gazette, excellent step coverage cannot be attained dissimilarly to the H.sub.2 reduction process, since the WF.sub.6 gas is reduced with the silane-system reducing gas.